The invention relates primarily to the preparation of polymers that are useful as precursors to ceramic materials, i.e., which serve as "preceramic" polymers, by catalytic activation of Si--H bonds, or as high temperature resins. Preceramic polymers are polymers which may be converted upon pyrolysis to ceramic products. The present invention provides preceramic silane, silazane, siloxane and carbosilane polymers which are useful for preparing a wide variety of silicious ceramic materials and articles, e.g., fibers, films, shaped products and the like, comprising materials such as silica, silicon oxynitride or silicon carbide.
In general, preceramic silicon-containing polymers, or "ceramic precursors," may be prepared by catalytic activation of Si--H bonds, and/or Si--N bonds, as disclosed in commonly assigned U.S. Pat. Nos. 4,788,309, issued Nov. 29, 1988, to Laine et al., 5,055,431, issued Oct. 8, 1991, to Blum et al., and 5,128,494, issued Jul. 7, 1992 to Blum, the disclosures of which are hereby incorporated in their entirety by reference. Briefly, silicon-containing starting materials containing Si--H bonds, and/or Si--N bonds, are reacted with a compound of the general formula R--X--H, wherein X is typically O or NH, and wherein R is H, alkyl or aryl, a moiety containing an unsaturated carbon--carbon bond, an amine or an organic or hydroxy metal compound.
Previously, catalytic activation of Si--H bonds was primarily used for hydrosilylation of unsaturated compounds, as illustrated by reaction (1): ##STR1## Numerous homogeneous and heterogeneous catalysts have been used to promote such reactions. See, e.g., J. L. Speier et al. (1957) J. Am. Chem. Soc. 79: 974. Typical application of these reactions has been in organic synthesis or in the crosslinking of silicon rubbers (J. P. Collman et al. in "Principles and Applications of Organotransition Metal Chemistry", pp. 384-392. University Science Books, 1980). Such reactions have been used in crosslinking of preceramic polymers, as described in commonly assigned U.S. Pat. No. 5,008,422, issued Apr. 16, 1991, to Blum et al., the disclosure of which is hereby incorporated in its entirety by reference.
Related reactions involving substitution at an Si--H bond have been used to form compounds containing Si--X groups wherein X is, for example, halogen, alkoxy, or substituted or unsubstituted amino: ##STR2## L. H. Sommer et al. (1967) J. Org. Chem. 32: 4270. Only mono- and di-substituted aminosilanes, halosilanes and alkoxysilanes have been synthesized by this method.
To date, many conventional methods of preparing ceramic precursors, and the precursors prepared thereby, suffer myriad problems including low ceramic yields and slow cure rates. High ceramic yields are of considerable value in binder applications, fabrication of injection molded parts and in matrix applications. During pyrolysis the density/volume change from preceramic polymer (1-1.3 g/cc) to amorphous ceramic (about 2.0 g/cc) can be significant. Thus, ceramic yields far below theoretical will only magnify the resulting density/volume change. For example, a 50% ceramic yield for a Si.sub.3 N.sub.4 precursor of density 1.0 will result in a final decrease in volume of approximately 80%. In general, preceramic polymers known in the art provide relatively low ceramic yields upon pyrolysis.
Synthesis of preceramic polymers generally includes a curing interval during which a preceramic composition must "set" before being amenable to pyrolysis to a ceramic material. This curing period can be slow, incorporate undesired elements such as excess oxygen and can significantly reduce the efficiency of the polymer processing method.
In addition, products of known synthetic methods are often undesirably contaminated with oxygen, are not readily machinable or otherwise workable and either cure too fast or too slowly. These disadvantages preclude the desired control of the preceramic polymer molecular weight, structural composition and viscoelastic properties which to a large extent determine the tractability of the polymer, the ceramic yield, and the capability for specific ceramic processing.
In addition to addressing the above-described disadvantages of the prior art, the present invention is directed to a new approach to polymer processing and involves preparation of preceramic polymers useful in making ceramic materials and reaction of polysilanes, polysiloxanes, polysilazanes and polycarbosilanes by catalytic activation of Si--H bonds contained therein. Preceramic polymers produced using the present method are highly "processable" and, upon pyrolysis, give the desired ceramic material in relatively high yield. The ceramic yield upon pyrolysis of ceramic precursor polymers produced as herein described is increased over that of previous polymers.
The invention also provides a unique and novel method of functionalizing silicon-containing preceramic polymers before, during or after curing. Previously, in order to obtain a preceramic polymer with specific functionalities, precursor monomers with the desired functional moieties were required as reactants in the polymerization process. The present invention represents a significant advance in the art by enabling functionalization of a basic preceramic polymer as desired, whether cured or not, rather than requiring de novo synthesis of a preceramic polymer possessing appropriate functional moieties. The invention provides an approach to preparing polymers containing functional groups such as NH.sub.2, OH, COOH or the like, that are also curable.
Advantages of the present invention include, but are not limited to, the following: the ability to modify preceramic precursors' rheological and pyrolytic properties; the ability to cure meltable preceramic polymers after fabrication without the need for conventional oxygen curing (oxygen curing produces oxides, thereby limiting ceramic fiber performance to about 1100.degree. C.); an increase in ceramic yield; and the ability to incorporate nitrogen in final ceramic products, thereby enhancing product performance.
Although the products of the method disclosed and claimed herein are referred to as preceramic polymers, the present invention can provide useful polymeric materials which are other than ceramic materials, i.e., polymeric materials which are useful without further conversion to a ceramic material.